1. Field of Invention
The present invention relates to a method for granulation of an adsorbent, and in particular, to a method for granulation of the adsorbent from sodium metasilicate, metakaolin and the inorganic ion exchange material to provide the adsorbent granules having excellent adsorbability and having an appropriate particle size. The present invention also relates to the adsorbent granules prepared by the method.
2. Related Art
With development of the nuclear industry, the radioactive wastewater is produced in great volume, causing serious damage to environment. For treatment of the radioactive wastewater, it primarily involves converting the high radioactive effluent by treatment to the moderate or low radioactive effluent, and solidifying it prior to low radioactive disposal, attempting to minimize the damage of the radioactive waste to environment as far as possible.
90Sr and 137Cs are the fission products with high heat release and long lifetime in the high radioactivity effluent, with a high ratio in the radioactive components, therefore for conversion of the high radioactive effluent into the low radioactive one, removal of 90Sr and 137Cs is a critical step. The conventional treatment methods and technologies comprise: concentration by evaporation, biological treatment, chemical precipitation etc., however in view of complicated composition of the actual waste water and presence of some unsolvable disadvantages for these technologies in practice, the ion exchange process becomes an increasingly popular treatment technology.
Separation of 90Sr and 137Cs from the high radioactive effluent by the ion exchange process is an important approach in the art, wherein an organic resin is a material commonly used in the conventional ion exchange technology, however, the organic resin is poor in heat resistance and radiation resistance, has high potential energy required for metal ion exchange, has voids readily formed during solidification, causing the effluent to elute out, and has the decomposed products which are not in favor of subsequent processing, compromising the treatment effects, therefore its value in practice hangs in doubt, and the inorganic ion exchange technology is preferred. Compared to the organic adsorbing materials, the inorganic adsorbing materials are advantageous as follows: (1) acid resistance; (2) high radiation resistance, useful for adsorptive separation under strong radioactivity; (3) high temperature resistance during final disposal of the radioactive waste, having good compatibility with glass and cement; (4) high selectivity; (5) simple to operate; (6) inexpensive for the natural inorganic ion exchange materials, and easy for preparation of the artificial inorganic ion exchange materials. Therefore, the inorganic ion exchange process is an economical and appropriate processing mode for treatment of the radioactive waste.
The inorganic ion exchange materials which has been used presently include primarily: (1) natural/artificial zeolite; (2) composite ion exchange materials, such as titanium phosphate Ti3(PO4)4 (TiP) complexed with ammonium phosphomolybdate (NH4)3PMo12O40 (AMP), zirconium phosphate Zr3(PO4)4 (ZrP) complexed with ammonium phosphomolybdate (ZrP-AMP), and stannum phosphate Sn2P2O7 (SnP) complexed with ammonium phosphomolybdate (SnP-AMP); (3) metallic ferrocyanide and ferricyanide; (4) heteropoly acid salts, ammonium phosphotungstate (APW), zirconium phosphotungstate (PWZr) and zirconium phosphomolybdate (PMoZr); (5) multivalent metallic phosphate; (6) multivalent metallic (transition metal) oxyhydroxides and hydrate.
The zeolite materials are a type of the firstly studied ion exchange materials. Its component is common aluminosilicate with strong radiation resistance and high temperature resistance, a three-dimensional crystalline essentially consisting of SiO4 and AlO4 tetrahedrons. Due to its high surface area, it has significant ion adsorption and exchange capacity, wherein its exchange capacity is significantly affected by acidity and salt content of the solution, with optimal working conditions of 3<pH<10.
The artificial zeolite enables removal of multiple radioactive ions, i.e., it enables not only removal of the mono- and divalent radioactive ions, but also effective removal of the radioactive rare earth ions, such as Y3+. Clinoptilolite has high cation exchange capacity, which is significantly selective for 137Cs over the other alkali and alkaline earth group element cations. The Na+ zeolite has a maximal distribution factor for 137Cs, and it is known from column test that the zeolite also has strong adsorption effect on 90Sr. Adsorption of Sr2+ on the zeolite includes predominant chemical adsorption and minor physical adsorption, and under varied acidity and salt load of the solution, adsorption capacity is also affected; at gradually increased pH, equilibrium adsorption capacity of zeolite for Sr2+ is also increased.
The modified natural zeolite and the artificial zeolite having high ion exchange capacity are of interest in study, wherein the zeolites of A type, synthesized from NaAlO2, Na2O.SiO2 and NaOH, exhibit its adsorption effect in the test for adsorption of Sr2+ and Cs+, the clinoptilolite of NaA-X type has super adsorption effect on Sr2+, and the synthetic zeolite of X type is significantly selective for monovalent and divalent ions.
AMP, ammonium dodecatungstophosphate (ATP), TiFCN, ZnFCN and CaFCN are the inorganic ion exchange materials having higher exchange capacity for Cs+, useful in an acidic simulation solution. Of these, Cs+ clearance is up to 95% for AMP, but due to in the form of fine powder microcrystallite structure, AMP has poor hydraulic performance, causing spray washing and column test to be difficult, resulting in its application to be limited.
Most of the composite ion exchange materials are limited severely in application due to difficult molding or irregular granules produced, poor mechanical strength and fragility in practice; therefore, the disadvantages above must be overcome prior to use in practice.
The composite TiP-AMP, which has been modified to have the preferred properties of particles, has superior performance over the ion exchange material having TiP or AMP alone, which allows blocking in TiP as the ion exchange material occurred during removal of 137Cs from the high radioactive effluent to be improved, and also solves the problem in granulation of AMP, leading to improvement of ion exchange performance, as indicated in the literature (Yu, B., J. Chen, X. W. Zhu and C. L. Song, Research progress of 137Cs removal from acidic high-level liquid waste. Atomic Energy Science and Technology, 2002. 36(1): p. 51-57).
[Fe(CN)6]4− has super high chemical stability, pore size, specific surface area, mechanical stability, radiation resistance, and less apparent density in favor of fluid transfer, and is an ion exchange material with high potential application. [Fe(CN)6]4− has strong binding to Cs+, and from the results in the literature, the strength preference for adsorption of the ions in the simulated high radioactive effluent onto the spherical [Fe(CN)6]4− is Cs+>>Ni+>Fe2+>Sr2+>Na+>Cr+>Nd+. However, for [Fe(CN)6]4− prepared by the general conventional methods, due to its irregular shape, too fine granule and poor mechanical strength, it is difficult to use in the column test.
Improvement of hydraulic performance of the granules was of much interest in recent years for the ferrocyanide ion exchange material, such as loading of cupric ferrocyanide onto polyurethane resin, or absorption of nickel potassium ferrocyanide onto zeolite, to prepare the composite ion exchange granules. In addition, preparation of the spherical composite ion exchange materials by sol-gel method has been also studied, including complexing the materials difficult to be molded by the conventional methods into or onto the gelled pellets. The granules prepared by this method are mostly in the form of ball with particle diameter being customized as demand, and has high mechanical strength, good flow property, uniformity, high porosity and specific surface area with high channel diameter, however the wet gelled pellets have shrunk significantly in volume during drying, and the gelled pellets of some composites are susceptible to fragile during drying.
Pyrophosphate as heteropoly acid salt is a type of high performance ion exchange materials, having exchange capacity for Cs+ over the commonly used inorganic ion exchange materials under neutral conditions. Zirconium pyrophosphomolybdate is an amorphous inorganic ion exchange material with high thermal stability and acid resistance, which keeps high ion exchange capacity in the acid medium, and is suitable for ion exchange with Cs+ under acidic condition. Pyrophosphate has a long chain in favor of improvement of material stability, and among various metallic ions, Sr2+ has a maximal distribution factor for stannum pyrophosphate, therefore stannum pyrophosphate is selected as ion sieve matrix for extraction of Sr2+. In the ion sieve for extraction of strontium, the voids for ion exchange have a size very close to Sr2+ ionic radius, causing chemical bond and sieving effect and improving the separation coefficient, and moreover, after physical process (such as granulation process) and chemical modification, it provides the sieving effect with significant selectivity, therefore the ion sieve for extraction of strontium would be expected to be a novel material for separation of radionuclide from the high radioactive effluent.
In the inorganic ion exchange materials, ZrP has strong affinity and high adsorbability for Cs, with exchange capacity of up to 4.3±0.1 meq g−1 in the basic solution, and exhibits favorable physical stability in both the dynamic and static ion exchange tests, and also has effective ion exchange properties at 300° C.
From the literature, in the process flow of recovering Cs+ from the acid waste water, both ZrP and TiP have favorable stability for radiation resistance, and the absorbed exchange materials are susceptible to regeneration by spray washing, and may be used for direct extraction of 137Cs from the acidic, high radioactive effluent, and also may be used as the adsorbent for gamma radiator. However, the phosphate exchange materials have low exchange capacity in the high radioactive effluent with acidity and high salinity, and it is difficult to allow the water quality of drain liquid to meet the standard for discharge water. Moreover, the circulating water for the reactor is usually neutral and basic, and ZrP is susceptible to loss of phosphate radical by hydrolysis, lowering ion exchange performance.
Most of oxyhydroxide and hydroxide of multivalent metals (transition metals) have amphoteric exchange property. Adjustment of pH of the alumina hydrate solution enables separation of the carrier-free radionuclides such as Fe, Mo, Tc and I. The quadrivalent metal oxides, such as SnO2, TiO2, ThO2 and ZrO2, also have the amphoteric exchange performance, and its specific boundary depends on alkalinity of center metal atom and intensity contrast of metal-hydrogen bond and hydrogen bond-hydrogen bond. MnO2 has significant adsorbability for fission product in the effluent; and for treatment of 89Sr-containing effluent by manganese oxide hydrate, its clearance may be up to 95%.
Oxyhydroxide and hydroxide of aluminum and ferrum also have significant absorptive effect on Sr2+, and in case of presence in the alkaline solution, have high adsorption effect on Sr2+, as a result of dissociation of H+ from carbonyl. Thermal treatment of oxide and hydroxide allows surface area to be substantially increased, leading to increased adsorption efficiency on Sr2+. However, the thermal treatment is controlled to be at 500° C. for 3 hours or more, without economical efficiency.
Furthermore, the surface area on the inorganic adsorbent is related with the capacity for adsorption of radioactive elements, but the flow rate of fluid must also be considered for ion exchange on column. In case of increased surface area of the adsorbent and reduced particle size of the adsorbent, it is easy to block the column due to relative reduction of channel space for fluid flow by dense packing in the column, potentially causing pressure loss in the adsorbing column to be too high. If the particle size of the adsorbent is increased so that the column is unblocked upon packing and has sufficient space in channel for fluid flow, the surface area of the adsorbent would be reduced, compromising the adsorbability.
To this end, how to provide the adsorbent granules with suitable particle size for packing but unblocking the column while keeping high absorption surface area to have adsorptive capacity, in a simple manner, is a challenge to be solved by the inventors. Therefore, in the extensive studies by the inventors for the methods for preparation and granulation of the inorganic adsorbent, it has been found that the adsorbent granules having suitable particle size and high surface area may be prepared by granulation of zeolite as the inorganic ion exchange adsorbent with sodium metasilicate and metakaolin, without heating, thereby implementing the present invention.